Document MongoDB get/setWriteConcern methods

https://jira.mongodb.org/browse/PHP-842


git-svn-id: https://svn.php.net/repository/phpdoc/en/trunk@332954 c90b9560-bf6c-de11-be94-00142212c4b1
Cross-reference MongoDB get/setReadPreference methods


git-svn-id: https://svn.php.net/repository/phpdoc/en/trunk@332955 c90b9560-bf6c-de11-be94-00142212c4b1